Walked slowly first along Tai Hang Road to Bowen road, GPS signal was low so app must be on the foreground, i.e. iPhone can't be in sleep mode. The distance was way out (1.62 vs 1.8) and no mapping due to low signal.
I then ran slowly 6k along Bowen Road, still too fast for the satellite, lots of trees and along hill side maybe the problem. Distance recorded was again way out (too much this time 7.61 vs 6). miCoach distance was dead on with only 0.6% error.
This $1.99 app may be OK elsewhere I'll go back to Nike+ and miCoach.
Ran between milestones of 6k A/MHR 151/157 score 80%